356 | Nature | Vol 584 | 20 August 2020
Perspective
with a higher prevalence of severe 2009 H1N1^42. Immunopathology
and C4d were reported in the lungs of six fatal cases in this age group,
indicating that antibody-dependent complement activation through
immune-complex formation may have been a contributing factor.
However, as noted above, other mechanisms lead to C4d deposition,
and lung T lymphocytosis attributed to T cell epitopes shared by 2009
H1N1 and earlier H1N1 strains was also observed, raising the possibility
that T cells played a part. Another study correlated pre-existing anti-
bodies that mediated infected cell lysis by complement activation with
protection against H1N1 in children^43. In a porcine model, enhanced
pulmonary disease was observed after vaccination with an inactivated
influenza H1N2 strain followed by heterologous H1N1 challenge^44. The
animals had non-neutralizing antibodies that bound haemagglutinin
in the stem region, but did not block the binding of haemagglutinin
to its cell receptor and accelerated fusion in vitro by a Fab-dependent
mechanism (Fig. 1 ). Lung pathology was also observed in mice treated
with a mAb that induced a conformational change in haemagglutinin
that facilitated fusion^45. Such a mechanism was postulated to have
potential clinical relevance when the infecting influenza virus has
undergone antigenic shift and the infection boosts non-neutralizing
haemagglutinin-stem-binding antibodies without a neutralizing
antibody response. The likelihood of these circumstances occur-
ring is unclear. Further, human influenza vaccines are not known to
elicit immunodominant antibodies with this property. Importantly, as
noted above, stem antibodies correlate both with resistance to infec-
tion and to severe disease in humans, indicating that this interesting
mechanism is not predictive of disease causation for stem-specific
antibodies^37. In addition, mAbs can be screened to avoid fusion-
enhancing properties, and fusion is not intrinsically accelerated by
low titres of neutralizing antibodies. Notably, infants benefit from
immunization from six months of age, despite their limited capacity
to produce affinity-matured, high-avidity antibodies. Overall, wide-
spread annual surveillance of influenza does not reveal ADE of disease,
even though cross-reactive strains and vaccine mismatches are
common.
Dengue
There are four viral serotypes of dengue that circulate in endemic
areas^19. Although severe dengue haemorrhagic fever and shock syn-
drome occurs during primary infection, possible ADE of disease has
been associated with poorly neutralizing cross-reactive antibodies
against a heterologous dengue serotype. Taking into account the dif-
ficulty of classification due to the overlapping signs of severe infection
and ADE of disease, clinical experience indicates that ADE of disease
does occur, but is rare in endemic areas (36/6,684 participants; around
0.5%) and is correlated with a narrow range of low pre-existing antibody
titres (1:21–1:80)^20. In the same study, high antibody titres were found
to be protective. The challenge of predicting how to avoid such a rare
immune-enhancing situation against the background of protection
conferred by dengue neutralizing antibodies implies that it will be
equally difficult for SARS-CoV-2.
When considering conditions that may result in ADE of disease,
it is important to emphasize that dengue differs from other viruses
because it targets monocytes, macrophages and dendritic cells and
can produce progeny virus in these cells, which abundantly express
both viral entry receptors and FcγRs. ADE of infection can be demon-
strated in vitro with FcγR-expressing cells—typically with cross-reactive
antibodies that have low or no neutralizing activity, have low affinity,
or target non-protective epitopes, or if a narrow range of antibody
and infectious virus concentrations is tested^46 ,^47. The mechanism of
ADE of disease associated with dengue therefore depends on three
factors: the circulation of multiple strains of a virus that have variable
antigenicity, a virus that is capable of replication in FcγR-expressing
myeloid cells and sequential infection of the same person with these
different viral serotypes. Despite these pre-disposing conditions and
the fact that dengue is an increasingly common infectious disease,
severe dengue disease is rare.
The role of pre-existing immunity has also been a concern for the
quadrivalent live attenuated dengue vaccine (Dengvaxia), because
higher hospitalization rates were observed among vaccine recipients
who were initially seronegative—especially children aged between two
and eight years^48. Other explanations for this outcome include poor
efficacy against serotypes 1–3, or the failure to induce cell-mediated
immunity because T cells primarily recognize non-structural proteins
that are not present in the chimeric vaccine. Importantly, the cause
of death in 14 fatal cases of dengue could not be determined by the
WHO (World Health Organization) Global Advisory Committee on
Vaccine Safety, because a failure of vaccine protection could not be
distinguished from immune enhancement by clinical or laboratory
criteria^49. This experience underscores how difficult it is to predict the
potential for vaccine-induced antibodies or a therapeutic antibody to
enhance the severity of disease, because other mechanisms of patho-
genesis that result in severe disease are potentially involved—even for
the well-studied case of dengue.
In other assessments of the risks and benefits of cross-reactive anti-
bodies, infection with Zika—which, as with dengue, is a flavivirus—was
less common in individuals who had previously been infected with
dengue^50. In addition, the presence of cross-reactive antibodies has
been associated with improved efficacy, as measured by the responses
to a yellow fever vaccine in recipients who had received a Japanese
encephalitis vaccine^47 , and by association of the effectiveness of Deng-
vaxia with seropositivity for dengue at the time of immunization^51.
In summary, these clinical experiences with RSV, influenza and den-
gue provide strong evidence that the circumstances that are proposed
to lead to ADE of disease—including low affinity or cross-reactive anti-
bodies with limited or no neutralizing activity or suboptimal titres—are
very rarely implicated as the cause of severe viral infection in the human
host. Furthermore, clinical signs, immunological assays or biomark-
ers that can differentiate severe viral infection from a viral infection
enhanced by an immune mechanism have not been established^49 ,^52.Assessing the risk of ADE of disease with SARS-CoV-2
Given the complexities described above, it is sobering to take on the
challenge of predicting ADE of disease caused by SARS-CoV-2. Here
we consider whether clinical circumstances point to a role for anti-
bodies with poor or no neutralizing activity in severe COVID-19, incor-
porating relevant experience from disease caused by the common
human coronaviruses, as well as by severe acute respiratory syndrome
coronavirus (SARS-CoV) and Middle East respiratory syndrome-related
coronavirus (MERS-CoV).
Infection by SARS-CoV-2 is initiated by the binding of its fusion pro-
tein, the spike (S) protein, to the entry receptor, angiotensin-converting
enzyme 2 (ACE2)^53 –^55. Other receptors for SARS-CoV-2, such as CD147,
have also been reported^56. ACE2 is expressed on alveolar type II pneu-
mocytes, airway epithelial cells, nasal tract goblet cells and ciliated
cells, as well as on intestinal and other non-respiratory tract cells, as
assessed by RNA expression^57. On most such cells, ACE2 seems to be
expressed at low levels; however, it can be upregulated by interferons^58 ,
which could theoretically promote infection if the virus overcomes
interferon-induced barriers. FcγRIIa and FcγRIIIa were detected in
alveolar, bronchial and nasal-cavity epithelial cells by single-cell RNA
sequencing, but both fractions of positive cells and levels of expression
per cell were considerably lower than for resident myeloid and natural
killer cells^59 ,^60. The moderate prevalence of both ACE2 and FcγRs results
in poor co-occurrence, although this might be underestimated because
of the dropout effect in single-cell transcriptomics. Co-expression of
ACE2 and FcγRs therefore seems to be limited, which would mitigate
against antibody-enhanced disease caused by SARS-CoV-2 via the
dual-receptor mechanism proposed in dengue infection.